The present invention relates to a regenerative braking system for a car, and particularly to a regenerative braking system for a car wherein the decelerating energy of a car is captured and used for the starting/accelerating energy.
In a PTO (Power-take-off) output unit (system) or a deceleration energy recovery system as conventionally known, a part of the kinetic energy dissipated mainly as heat at a brake or engine during the deceleration of a car is captured in the form of hydraulic operating oil and accumulated in an accumulator. The accumulated energy is utilized for the starting energy and accelerating energy of the car.
For example, in 1976 C. J. Lorence Corporation in England announced the development of such a system using a bus of British Leyland Corporation. Since then, various research and development has been conducted in Europe.
Recently, Japanese Patent Application Laid-open Nos. 62-15128, 62-37215, and 62-39327 have disclosed a deceleration energy recovery system which is essentially formed of a transmission (hereinafter referred to as T/M), a multi-stage gear-changed PTO unit, a PTO output shaft, a pump/motor, a hydraulic oil circuit, an electromagnetic clutch, an accumulator, and a control unit.
The T/M includes a counter shaft driven through an engine clutch, a main shaft connected to a wheel driving line, and a multi-staged gear train mechanism transferring the rotation of the counter shaft to the main shaft through the gears. The multi-stage gear-changed PTO unit includes a counter shaft PTO gear disconnectably coupled to the counter shaft through a counter shaft PTO gear synchronizer, a main shaft PTO gear coupled to the counter shaft PTO gear and disconnectably coupled to the main shaft through a main shaft PTO gear synchronizer, and a PTO output shaft driven through driving gears coupled to the main shaft PTO gear. The pump/motor is coupled to the PTO output shaft, the hydraulic oil circuit consists of a high pressure oil line and a low pressure oil line and serves to connect the accumulator to an oil tank through the pump/motor, and the electromagnetic clutch serves to connect/disconnect the oil circuit to/from the PTO shaft.
The control unit controls the electromagnetic clutch and works the pump/motor as either a pump or a motor in response to the running condition of the car. Namely, for working the pump/motor as a pump, the torque of the wheels during the decelerating mode serves to accumulate the operating oil into the accumulator through the PTO unit, thereby to capture the kinetic energy, i.e. braking energy mainly lost as heat in the brake or engine, and for working the pump/motor as a motor, the operating oil accumulated in the accumulator serves to generate starting/accelerating torque to drive the wheels through the PTO unit.
The control manner of this control unit is as follows: 1 When the car starts with the inner pressure of the accumulator being sufficient, the pump/motor is controlled to serve as a variable capacity type motor, the capacity of which is controlled by varing the displacement angle (incline angle) of the swash plate or shaft in response to the accelerator pedal positions.
Since the pump/motor is also connected to the electromagnetic clutch, when the electromagnetic clutch is coupled to the PTO unit by the control unit, the pump/motor drives the car based on the hydraulic power accumulated in the oil circuit.
In the meantime, when the car speed exceeds a preset value corresponding to the gear position selected by a driver, the engine clutch is coupled to the engine for the engine cruising. At the same time the PTO unit is gear-changed so as to turn off the counter shaft synchronizer which was on, and turn on the main shaft synchronizer which was off, and the combined crusing of the pump/motor with the engine is carried out according to the hydraulic power based on the pedal position only if the accelerator pedal is largely operated by the foot.
2 During the braking mode, the electromagnetic clutch is made on, and the displacement angle control signal (pump capacity control signal) according to the brake pedal position is supplied to the pump/motor for the corresponding pumping operation, while at the same time the engine is declutched.
Thus, the control unit controls the engine clutch to decouple the engine from the driving line of the wheels in order to capture a part of the braking energy which is to be consumed in the engine during the braking mode and to relieve the captured energy during the accelerating mode, while coupling the engine to the driving line in order to use the engine power solely or in combination with the motor during the starting/accelerating mode.
In such prior art structures, although the braking operation by the hydraulic power is necesarily performed by the pump/motor as long as the car speed is not null and the brake pedal is operated by the foot, it is disadvantageous to effect hydraulic braking and/or engine braking without taking measures to meet the case where the maximum hydraulic braking torque is smaller than the required braking torque and therefore a sufficient braking power cannot be obtained by a large operation of the brake pedal.
It is also disadvantageous that since a hydraulic braking operation is continued even for a low car speed, the hydraulic system is frequently used particularly when the car cruises at a low speed, such as in a traffic jam where the energy recovered is little, resulting in the deterioration of the durability of the system.